1. Field of the Invention
This invention relates to methods and apparatus for filtering liquids containing suspended solids. Specifically, this invention relates to an improved vertical filtration unit.
2. Background Art
The removal of suspended solids from liquids, especially water, has been a longstanding requirement for many uses of such liquids. For example, water which is free of suspended solids is greatly preferred for uses such as drinking, industrial processing and swimming pools. Vertical filters, which utilize filter media such as sand, have been used to separate suspended solids from water.
Vertical filters operate by introducing the liquid containing suspended solids onto the top portion of a bed of filter media, usually sand. As the liquid passes through the filter media under the force of gravity, the suspended solids adhere to the filter media, allowing liquid free of suspended solids to accumulate and exit from the bottom of the vertical filtration unit.
The above-described method, although effective at removing suspended solids from liquids, has several shortcomings.
The filter media eventually becomes saturated with suspended solids and no longer effectively removes suspended solids from the incoming liquid.
When the condition of (A) occurs, the vertical filtration unit must cease operation so that the captured suspended solids can be removed from the filter media. This removal is accomplished through an operation referred to as "backwashing," in which clean liquid is introduced, under pressure, to the bottom of the filter media bed and passes through the filter media in the reverse direction from the filtration process. The reverse flow of the liquid removes the filtered solids from the filter media and exits through the top of the vertical filter. This operation is typically continued until the exiting backwash water is free of suspended solids, indicating that the filter media is clean.
Backwashing can be very time consuming and generates large volumes of wastewater.
Filtered solids are never completely removed from the filter media during backwashing which eventually results in the filter media needing to be replaced.
Filtration devices utilizing the above method are disclosed in U.S. Pat. No. 650,611.
The problem of backwashing filter media has been addressed, to some extent, through the development of continuous, or moving bed filtration systems. While these continuous filtration systems operate to remove suspended solids as outlined above, they have the added benefit of continuously removing filtered suspended solids from the filter media. In these methods, the dirtied filter media at the bottom of the vertical filter unit and a small amount of filtered water are airlifted to a washbox. This lifting is carried out by an airlift tube equipped with a compressed air supply. The mixture of filtered water, filtration sand and suspended solids is directed against a baffle or plate to scrub the suspended solid particles from the sand.
The washed sand is returned to the top of the filter media bed. The suspended solid particles, washed from the filter media, are flushed from the filter by a combination of filter effluent liquid and filtered liquid that carry the suspended solids across a reject weir and out of the vertical filtration unit. The filtration cycle automatically repeats itself with the influx of a new supply of liquid to be filtered.
Examples of the above-described continuous filtration systems are disclosed and explained in U.S. Pat. Nos. 4,060,484, 4,891,142 and 5,582,722, all of which are herein incorporated by reference.
The above-described continuous systems for the filtration of liquids and the removal of suspended solid particles are typically not as effective or efficient as industry and the public need and desire. These systems typically require level sensors and valve actuators which are expensive and fail when electrical power supply is lost. Many valves are often required which can cause confusion for those operating the filtration unit. The airlifting operation often "plugs" with filter media, causing the filtration unit to be shut down for cleaning. There remains an unmet need to provide a reliable, maintenance-free, continuous, self-cleaning, vertical filtration unit for the removal of suspended solids from liquids.